dy_import_module_symbols("random.r2py")

# See RFC 2030 (http://www.ietf.org/rfc/rfc2030.txt) for details about NTP
# this unpacks the data from the packet and changes it to a float
def convert_timestamp_to_float(timestamp):
  integerpart = (ord(timestamp[0])<<24) + (ord(timestamp[1])<<16) + (ord(timestamp[2])<<8) + (ord(timestamp[3]))
  floatpart = (ord(timestamp[4])<<24) + (ord(timestamp[5])<<16) + (ord(timestamp[6])<<8) + (ord(timestamp[7]))
  return integerpart + floatpart / float(2**32)


def decode_NTP_packet(ip, port, mess, sockobj):
  log("From " + str(ip) + ":" + str(port) + ", I received NTP data.\n")
  log("NTP Reference Identifier:", mess[12:16], '\n')
  log("NTP Transmit Time (in seconds since Jan 1st, 1900):", convert_timestamp_to_float(mess[40:48]), '\n')
  sockobj.close()

timeservers = ["time-a.nist.gov", "time-b.nist.gov", "time-a.timefreq.bldrdoc.gov",
               "time-b.timefreq.bldrdoc.gov", "time-c.timefreq.bldrdoc.gov",
               "utcnist.colorado.edu", "time.nist.gov", "nist1.symmetricom.com",
               "nist.netservicesgroup.com"]

# choose a random time server from the list
servername = timeservers[int(randomfloat()*len(timeservers))]
log("Using: ", servername, '\n')
serverip = gethostbyname(servername)

# this sends a request, version 3 in "client mode"
ntp_request_string = chr(27)+chr(0)*47

ip = getmyip()
port = 34612

# binds to an IP and port and waits for incoming UDP messages
udpserversocket = listenformessage(ip, port)

sendmessage(serverip, 123, ntp_request_string, ip, port) # port 123 is used for NTP

while True:
  try:
    remoteip, remoteport, message = udpserversocket.getmessage()
    decode_NTP_packet(remoteip, remoteport, message, udpserversocket)
  except SocketWouldBlockError:
    sleep(0.1)
  except SocketClosedLocal:
    break